The present invention relates to a device for detecting rotation of a rotary shaft, and more particularly to a device for detecting the rotating angle or rotating torque thereof, and also relates to a rotary shaft controlling apparatus such as an electric power steering apparatus using a rotating torque detecting device.
In order to control the rotation of the device provided with a rotary shaft, such as the steering shaft for an automobile and the rotating arm of a robot or a machine tool, it is necessary to detect the rotating angle or rotating torque of the rotary shaft.
The device for detecting the torque of a rotary shaft is disclosed in e.g. Japanese publication JP-A-1-97826 corresponding to the Japanese application filed by HITACHI, LTD. on Oct. 9, 1987. In the torque detecting device disclosed in this publication, the rotating torque is calculated using rotating angle sensors located at two points of a rotary shaft which are spaced from each other as follows. First, two magnetoresistance (MR) devices arranged in the neighborhood of a magnetic drum detect the magnetic field generated when the peripheral surface of the rotating drum coaxially connected with the rotary shaft is magnetized alternately with an N pole and an S pole. The rotating angle sensors detect the rotating angles of the rotary shaft from a sine signal and a cosine signal generated by the two MR devices, and further detect the difference in the rotating angles due to twists at two points of the shaft when the rotary shaft is rotated, thereby calculating the rotating torque. More specifically, as signals are generated the amplitudes of the signals change in the form of a sine wave and a cosine wave owing to a change in the rotating angle of the shaft; the value of the sine wave is divided by the value of the cosine wave to provide a tangent function x=tan .theta., and the arc-tangent function (.theta.=tan .sup.-1 x) of the tangent function is calculated to provide the rotating angle .theta..
U.S. Pat. No. 4,774,464 issued to Masanori Kubota on Sep. 27, 1988, discloses an example of a rotating sensor using a magnetic drum and an MR sensor attached to a rotary shaft. Such a rotating angle sensor using an MR device is excellent in endurance and reliability because it has no slidable portion (i.e. is of a non-contact type).
Examples of a rotating angle sensor and a rotating torque detecting device are also disclosed in JP-A-62-6130 filed by HITACHI, LTD. on Oct. 9, 1987, JP-A-62-67401 filed by KOBE SEIKO CO., LTD., JP-A-64-265127 filed by HITACHI, LTD. on Apr. 23, 1987, U.S. Pat. No. 4,724,710 issued to Murty on Feb. 16, 1988, and U.S. Pat. No. 4,506,554 issued to Blomkvist, et al on Mar. 26, 1985. Further, examples of the electric power steering device are disclosed in U.S. Pat. No. 4,828,060 issued to Drutchas, deceased et al on May 9, 1989, and U.S. Pat. No. 4,828,061 issued to Kimbrought et al on May 9, 1989.
Now it should be noted that the rotation sensor and the rotation torque sensor using the rotation sensor hitherto known have no means for obviating output error. The output error is due to a change in the offset voltage of an operational amplifier for amplifying a minor signal from a sensor device, a change in the signal amplitude, inequality in the magnetization amount on a magnetic drum, and a change in the relative position between the MR device and the magnetic drum. These causes of error lead to changes in the central voltage of the output waveform from the MR device and in the phase difference in the two output waveforms, which in turn lead to error in the rotating angle and rotating torque to be detected.
For example, in the case where a rotation sensor is used for an electric power steering device, the presence of a small torque ripple due to a torque detection error in an electric motor generating an auxiliary steering torque gives a driver a sense of unpleasantness and anxiety. To obviate this, it is necessary to provide a signal adjusting circuit for correcting the offset value or the amplitude value individually from the sensor. However, this makes the control circuit complicated and therefore the adjustment is troublesome.